Research and development on magneto-optical disks as rewritable optical disks are being carried out actively, and some magneto-optical disks have been practically used as external memory for computers.
A magneto-optical disk uses a layer having a perpendicular magnetization as a recording medium, and records and reproduces information using light. The magneto-optical disk having such structures has an increased recording capacity compared with a floppy disk and a hard disk which use a layer having an in-plane magnetization.
As illustrated in FIG. 12, a magneto-optical disk is provided with grooves 51 so that a light spot 55 accurately follows a land 52 formed between the grooves 51. Address information is recorded in the form of pits 53 on each of the lands 52 so that the address information of a track being scanned by the light spot 55 is obtained.
Information is recorded on and reproduced from tracks as lands 52. The track pitch is almost equal to the diameter of the light spot 55 which varies depending on the wavelength of laser light and the numerical aperture of an objective lens. The objective lens converges the laser light into the light spot 55. Usually, the wavelength of the laser light is between 780 nm and 830 nm and the numerical aperture of the objective lens is between 0.45 and 0.6. Thus, the diameter of the light spot 55 is set between 1.2 .mu.m and 1.4 .mu.m and the track pitch is set between 1.4 .mu.m and 1.6 .mu.m. Accordingly, the minimum diameter of an upwardly or downwardly magnetized recording domain 54 is around 0.8 .mu.m.
A magneto-optical disk having flat mirrored sections 62 shown in FIG. 13 is also well known. Although pits 63. are formed in the mirrored sections 62, the mirrored sections 62 do not have grooves 61. The light spot 55 tracks the grooves 61, and the address information of a track being scanned by the light spot 55 is obtained by reproducing the pits 63. Similarly to the above-mentioned optical disk of FIG. 12, the minimum diameter of a recording domain 64 on the groove 61 of this magneto-optical disk is around 0.8 .mu.m.
In recent years, some methods have been proposed to increase the recording density of a magneto optical disk by using a recording film composed of magnetic layers and reproducing a recording bit of a size much smaller than the size of the light spot 55 by magnetically induced super resolution. For example, detailed descriptions of such methods are in the following documents: Japanese Publication for Unexamined Patent Application No. 81717/1993, and "Sony Magnetically Induced Super Resolution Magneto-Optical Disk", Journal of Magnetics Society of Japan, Vol. 15, No. 5, 1991, pp. 838-845.
With such a structure, since a recording bit whose size is almost a half of the above-mentioned bit size is reproducible, it is possible to reduce the track pitch to around 0.8 .mu.m, i.e., one half of the conventional track pitch.
With the conventional structure, however, when the track pitch is reduced to a half, the distance between the pits 53 formed in adjacent tracks is also decreased to a half. This causes crosstalk, and prevents accurate address information from being obtained.
On the conventional magneto-optical disk, information is recorded on either the grooves or the lands. This structure imposes a limit to high-density recording because either the grooves or the lands are not used for recording.